Their relationship can be developed through shared adventures. For example, going on quests where they rely on each other's skills. Rex might need Mythra's combat abilities and Mythra could need Rex's determination and kindness.
For Rex, you could develop his character by showing his more vulnerable side when he's around Breach. Maybe he's usually the tough leader, but with Breach, he allows himself to show his doubts and fears. For Breach, show how he matures through his interactions with Rex. For example, he could become more responsible.
One idea could be that Mythra and Rex have to team up to save their world from a new, powerful enemy. During their journey, they discover new powers within themselves and also realize their feelings for each other are deeper than they thought.
Their relationship can be developed through shared experiences. For example, if they are in a battle together, Rex protecting Ahsoka and Ahsoka using her Jedi powers to save Rex. This would create a sense of mutual respect and care.
The relationship can be developed through mutual need. For example, if the human is lost in the dinosaur's territory, the T - Rex could initially see the human as prey but then realize the human has some unique skills, like making fire or using tools. This could lead to a truce. Or if the T - Rex is injured, the human could use their medical knowledge (assuming some sort of knowledge transfer in the story) to help the T - Rex, and in return, the T - Rex protects the human from other threats.
Their relationship can be developed through shared experiences. For instance, when they fight side by side against a common enemy. This would create a bond of trust and mutual respect between them.
Perhaps a near - death experience where Odd saves Aelita or vice versa. This would create a strong emotional connection between them. After that, they would be more protective of each other and their relationship would grow into a more intimate friendship or even something more romantic over time.
To develop their relationship, you could show them in normal, everyday situations. Like them having coffee together and just talking about their dreams and fears. This would humanize them and make their relationship more relatable. Also, having them save each other's lives in different scenarios would strengthen their bond. Maybe Spiderman saves Gwen from a falling building one time, and then Gwen saves him from a dangerous chemical spill another time. And then there could be moments of jealousy when another character shows interest in either of them, which would make them realize their true feelings for each other.
Their relationship could start with a shared goal, like protecting a particular person or place. Through working together towards this goal, they get to know each other better.
In a 'human x slime girl' fanfic, the relationship can start with curiosity. The human might be intrigued by the slime girl's appearance or abilities, and the slime girl could be interested in human culture. As they spend more time together, they start to care for each other. They could also have moments of vulnerability, like when the slime girl is in a difficult situation where her slime - like nature is a disadvantage, and the human helps her. This would strengthen their bond. Moreover, small acts of kindness, like the human sharing food with the slime girl (even if she has a different way of 'eating'), can be a step in developing their relationship.
Their relationship can be developed by having them train together. Naruto can share his experiences and wisdom gained from his own training and battles. Konohamaru can then look up to Naruto even more and try to implement those teachings in his own ninja way.